Although flexible, high strength, lightweight, tubular rods are desirable for a wide variety of usages, such structural configurations are particularly suited for fishing rods.
Over the past three decades numerous materials and designs have been employed in the construction of fishing rods, but the use of reinforcing filaments embedded in a hard, but flexible, resin have been particularly satisfactory.
Initially, such rods were fabricated with fiberglass strands embedded in resin. One of the pioneer patents in this area--U.S. Pat. No. 2,571,717--taught the use of longitudinally oriented, resin coated, fiberglass strands laid over a mandrel and wrapped with a cellophane tape until the resin cured. Another approach--as disclosed by U.S. Pat. No. 2,726,185--was to wrap a resin impregnated fabric comprised of fiberglass reinforcing strands onto a mandrel and allow the resin to harden with the fabric so wrapped upon the mandrel. Both of these prior art patents emphasized the longitudinal orientation of the fiberglass strands to impart the necessary flexural strength to the finished rod.
U.S. Pat. No. 2,749,643 contributed a further advance to the art by teaching that the reinforcement achieved by the longitudinally oriented fibers would be augmented by the use of a short pitch helical wrap of fiberglass strands to impart hoop strength to the arrangement. The aforesaid prior art, however, taught that all reinforcing strands in any one rod would be of a common material.
Eventually rods were made which constituted a composite of more than one type of reinforcing strand. U.S. Pat. No. 4,061,806 exemplifies the use of an intermix of glass fibers and graphite fibers to provide a very lightweight, high strength construction.
Even over and above lightness of weight and high strength, the "action" is perhaps one of the most significant characteristics of a fishing rod.
A typical hollow fishing rod made only of glass strands encapsulated in a suitable resin normally would have a modulus of elasticity on the order of 5.times.10.sup.6 psi. A comparable rod made only of graphite encapsulated in a suitable resin would have a modulus of elasticity on the order of 12.times.10.sup.6 psi. Hence, the graphite rod would be stiffer than its fiberglass counterpart. Although each fisherman is somewhat of an individualist, a great number appreciate the stiffer response through the body portion of the rod but prefer a more flexible tip portion.
Some initial experiments were made in employing materials of different moduli in the body and tip portions, but those attempts effected joinder between the different sections of the rod by such means as ferrules or adhesive butt-joints. Such joinder techniques effect a sharp demarcation between portions of the rod having different flexural characteristics.
When such a rod is bowed, as when playing a fish, the different flexural characteristics on either side of the joinder impart an appearance that the rod has some inherent structural flaw which is about to permit the rod to break under load. This would be aesthetically unacceptible even if the rod were functionally unimpared.
However, such a joinder is marked by a localized increase in the diameter of the rod which not only detracts from the aesthetic appearance thereof but also functionally affects stress concentrations across the joinder that are highly deleterious to the integrity of the rod.